Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1999-03-01
2002-04-23
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S095000, C521S135000, C521S178000
Reexamination Certificate
active
06376564
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to storage-stable foamable compositions based on epoxy resins. More specifically, specific combinations of blowing agents and curing accelerators are utilized to obtain resins which exhibit improved stability at room temperature over prolonged periods of time, yet can be heated and cured to provide foams capable of strengthening and stiffening hollow metal parts and the like.
2. Discussion of the Related Art
It is known that a number of industries, e.g., the automobile industry, require parts that are both strong and light-weight. One attempt to achieve this balance between strength and minimal weight provides for hollow metal parts. However, hollow metal parts are easily distorted. Accordingly, it is also known that the presence of structural foam in the cavities of the hollow parts can improve the strength and stiffness of such parts.
Generally, such foams comprise a thermosettable resin such as an epoxy resin, a blowing agent and a filler. Preferably, these foams have a density of about 20-40 lb/ft
3
(about 0.30-0.65 g/cc) and are able to withstand heat in excess of 175° C., more preferably in excess of 200° C. Optional ingredients include accelerators, curing, agents, processing aids, stabilizers, colorants, and UV absorbers.
Specific formulas for structural foam can vary widely. For example, U.S. Pat. No. 5,575,526 teaches several resin-based structural foams including Formula 2, which contains 54.5% EPON 828 epoxy resin, 7.5% HALOXY 62 epoxy diluent, 6.1% DER 732 flexible epoxy, 2.0% EXPANCEL 551DU blowing agent, 8.8% MICROS microspheres, 17.7% 3M K20 microspheres and 3.4% DI-CY dicyandiamide curing agent. U.S. Pat. No. 5,755,486 discloses thermally expandable resin-based materials containing, for example, epoxy resin, acrylonitrile-butadiene rubber, calcium carbonate, carbon black, fumed silica, glass spheres, curing agent, accelerator, and blowing agent. Modified ureas and blocked boron trichloride complexes are taught to be suitable for use as accelerators. Structural foams such as, e.g., TEROCORE® (a product of Henkel Surface Technologies), are now used in a variety of industries.
One recurrent problem with many of the structural foam formulations of this type that have been developed to date, particularly the one part formulations, has been their relatively short shelf-life.
Ideally, the components of such formulations once combined together should remain stable and unreacted under the conditions normally encountered during storage and shipment. It would be highly desirable for the properties of the formulations to be unchanged when stored at temperatures at or somewhat above room temperature over a several week or several month period. Unfortunately, certain of the presently available structural foam products tend to partially expand and cure at a temperature of 130° F. (a temperature which storage facilities and transportation means frequently exceed during the summer months or in warm climates) within 24 hours. Merely lowering the amount of curative is not the solution to the problem, since this will typically adversely affect the ability to cure and foam the resin when desired within commercially acceptable temperature and time limits.
SUMMARY OF THE INVENTION
It has now been surprisingly discovered that the shelf-life problems inherent in previously known structural foam formulations based on epoxy resins may be effectively alleviated through the use of amine-epoxy adduct curatives and an azodicarbonamide blowing agent. Storage-stable foamable compositions thus are provided which are comprised of one or more epoxy resins, one or more blowing agents (at least one of which must be an azodicarbonamide), and one or more curatives (at least one of which must be an amine-epoxy adduct). In one especially advantageous aspect of the invention, the foamable composition is in the form of a pliable dough which additionally contains one or more fillers, particularly hollow glass microspheres. Such compositions when foamed and cured furnish reduced density foams having excellent physical properties, including high strength and stiffness.
DETAILED DESCRIPTION OF THE INVENTION
Any of the thermosettable resins having an average of more than one (preferably, two or more) epoxy groups per molecule known or referred to in the art may be utilized as the epoxy resin component of the present invention.
Epoxy resins are described, for example, in the chapter entitled “Epoxy Resins” in the Second Edition of the
Encyclopedia of Polymer Science and Engineering
, Volume 6, pp. 322-382 (1986). Exemplary epoxy resins include polyglycidyl ethers obtained by reacting polyhydric phenols such as bisphenol A, bisphenol F, bisphenol AD, catechol, resorcinol, or polyhydric alcohols such as glycerin and polyethylene glycol with haloepoxides such as epichlorohydrin; glycidylether esters obtained by reacting hydroxycarboxylic acids such as p-hydroxybenzoic acid or beta-hydroxy naphthoic acid with epichlorohydrin or the like; polyglycidyl esters obtained by reacting polycarboxylic acids such as phthalic acid, tetrahydrophthalic acid or terephthalic acid with epichlorohydrin or the like; epoxidated phenolic-novolac resins (sometimes also referred to as polyglycidyl ethers of phenolic novolac compounds); epoxidated polyolefins; glycidylated aminoalcohol compounds and aminophenol compounds, hydantoin diepoxides and urethane-modified epoxy resins. Mixtures of epoxy resins may be used if so desired; for example, mixtures of liquid (at room temperature), semi-solid, and/or solid epoxy resins can be employed. Any of the epoxy resins available from commercial sources are suitable for use in the present invention. Preferably, the epoxy resin has an epoxide equivalent molecular weight of from about 150 to 1000. The use of epoxy resins based on glycidyl ethers of bisphenol A is especially advantageous. The epoxy resin(s) preferably have an average of about 2 epoxy groups per molecule. The epoxy resin or resins should be selected so as to provide the desired combination of properties in both the foamable dough and the final cured foam.
Selection of the blowing agent system to be employed is critical to the attainment of a storage-stable foamable composition. Azodicarbonamide (also sometimes referred to as 1,1′-azobisformamide, AZDC, or ADC) must comprise at least part of the blowing agent system. Preferably, the blowing agent system is predominantly (i.e. >50% by weight) azodicarbonamide. Even more preferably, at least 80% by weight of the total amount of the blowing agent system is azodicarbonamide. Blowing agents such as sulfonyl hydrazides may be used as minor components of the blowing agent system. In one embodiment, however, azodicarbonamide is essentially the only blowing agent present in the foamable composition. Azodicarbonamide is available from a number of commercial sources; for example, it is sold under the trademark UNICELL by Dong Jin Chemical of South Korea and under the CELOGEN trademark by Uniroyal Chemical. The particle size of the azodicarbonamide may be adjusted so as to provide the desired foaming characteristics in the foamable composition. For example, it has been found that relatively small particle size azodicarbonamides tend to produce foams having more uniform cell structure than coarser grade azodicarbonamides. “Activated” or “modified” forms of azodicarbonamide may be used to advantage in the present invention.
In some formulations, it may be desirable to also utilize a blowing agent activator or accelerator so as to lower the temperature at which release of the gas from the blowing agent takes or to increase the rate at which gas is released at a given cure temperature. Typically, from about 0.1% to about 2% blowing agent accelerator/activator based on the weight of the foamable composition is employed, although the optimum amount will of course vary depending upon the activator/accelerator selected, the amount of blowing agent, cure temperature and other variables. Excess activator/accelerator s
Foelak Morton
Harper Stephen D.
Henkel Corporation
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